3 Blisseys
Will a team of 3 blisseys ever be a thing, or 3 chanseys for great league, or maybe 3 mewtwos in the ultra league... Or something like that, maybe not 3 but two plus a counter for blissey, or teams of 3 counters of blissey to fight the 3 blisseys? Or can't you have three equal pokemons in your team?
Answers
Mewtwo has a lot of flexibility now in moveset given that they gave it access to three new charge moves a couple months ago and there will be a way to unlock a second charge move slot. Thunderbolt/Ice Beam and Shadow Ball/Focus Blast are potent combos. The latter in particular hammers pretty much all of Mewtwo's natural counters, hits Blissey hard and hits Metagross hard.
"Or can't you have three equal pokemons in your team?"
This is exactly what I'm thinking. Why do people think we'll be able to use the same species for a team? Species clause exists in the main series when battling other people so I'd imagine it'd be the same here. Didn't they highlight the fact that a variety of pokemon would be useable? That wouldn't work if they allowed a team of 3 Mewtwos or Blisseys.
Blissey is the strongest neutral TDO at 2500CP, but with resistances and SE, there are pokemon that beat it.
Metagross being the obvious choice - with a double resistance to psychic and resistances to fairy and normal, Blissey will be doing much less damage with any moves. A single Metagross should actually be enough to fully outclass the 3 Blissey Strategy.
Now, 2 Metagross + 1 Blissey may be a strong combo. It obviously loses to 3 Metagross, but it may be able to beat more teams than a 3 Metagross team can beat.
At the end of the day, no one strategy can ever be the best, because there are counters to any pick.